Method of conditioning the surface of acrylonitrile-butadiene-styrene

ABSTRACT

A TREATMENT FOR ABS POLYMERS TO IMPROVE ADHESION OF COATINGS THERETO. IMPROVED WETTABILITY OF THE SURFACE RESULTS FROM THE OXIDATION OF A PORTION OF THE AVAILABLE DOUBLE BONDS ON THE SURFACE OF THE POLYMER. THE POLYMER IS TREATED IN A SOLUTION CONTAINING FROM 50 TO 80 PERCENT BY VOLUME OF U.S.P. GRADE ORTHOPHOSPHORIC ACID (85.3% H3PO4) AND FROM 5 GRAMS PER LITER TO SATURATION OF POTASSIUM PERMANGANATE.

United States Patent ()1 fice 3,598,639 Patented Aug. 10, 1971 3,598,630 METHOD OF CONDITIONING THE SURFACE OF ACRYLONITRILE-BUTADIENE-STYRENE Warren R. Doty, Royal Oak, and Bobbie D. Knight,

Tecumseh, Mich., assignors to General Motors Corporation, Detroit, Mich. No Drawing. Filed Dec. 22, 1967, Ser. No. 692,702 Int. Cl. B44d ]/092 US. Cl. 117-47 4 Claims ABSTRACT OF THE DISCLOSURE A treatment for ABS polymers to improve adhesion of coatings thereto. Improved wettability of the surface results from the oxidation of a portion of the available double bonds on the surface of the polymer. The polymer is treated in a solution containing from 50 to 80 percent by volume of U.S.P. grade orthophosphoric acid (85.3% H PO and from 5 grams per liter to saturation of potassium permanganate.

This invention relates generally to the treatment of plastics to render them more susceptible to the receipt and retention of coatings applied thereto. This invention is more particularly directed to the decorative plating of acrylonitrile butadiene styrene.

With the recent advances in the molding of such commercially attractive polymers as acrylonitrile butadiene styrene (ABS) has come a considerable effort by the industry to improve the coatability thereof. A variety of techniques, including mechanical, electrical and chemical, have been developed which improve the adhesion of coatings applied to the surfaces of these polymers. Among the techniques which have become commercially significant in conjunction with the treatment of ABS polymers are the sulfuric acid family of etchants. These can generally be classified into three principal categories. The first category is the high concentration sulfuric acid treatment wherein solutions containing about 70% by volume of sulfuric acid and about 20-25 grams per liter of CrO are applied to the surface. The next category is the intermediate concentration sulfuric acid treatment wherein solutions containing about 45% by volume of sulfuric acid, 22% by volume of phosphoric acid and about 30 g./l. of CrO are applied to the plastic surface. The last category is the low concentration sulfuric acid treatment wherein solutions containing about 20% by volume of sulfuric acid and about 585 g./l. of CrO are applied to the plastic surface. All of these solutions generally require operating temperatures of about 5070 C., and are highly corrosive. Attempts have been made to substitute other oxidizing agents such as KMnO; in place of the CrO in the sulfuric acid family of solutions. These attempts have met with virtually no commercial success owing to the instability of the KMnO in H 80 The high decomposition rate of the permanganate in the sulfuric acid solution resulted in solutions having extremely short useful lives. Sulfuric acid solutions of potassium permanganate, for the most part, have practical service lives of less than 1 day. Attempts to revitalize the solutions by adding KMnO thereto were frustrated when manganese dioxide formed. The manganese dioxide appeared to catalyze the autodecomposition of the permanganate and accelerate the overall breakdown rate of the bath.

It is, therefore, an object of this invention to provide an improved stabilized acidified KMnO solution for the treatment of ABS surfaces. It is a further object of this invention to provide a surface treatment for ABS which can be carried out at room temperature. These and other objects and benefits of this invention will become more clear from the detailed discussion which follows.

We have discovered an improved surface conditioner for ABS plastics which comprises a solution of potassium permanganate in phosphoric acid. Our solution not only insures that the surfaces treated will be completely and adhesively covered by the coatings applied thereto, but the solutions themselves are usefully sustainable for commercially practical periods. Our solution is particularly useful to the industry owing to its capability of operating at about room temperature.

The degree and character of surface conversion varies with the strength of the solution, treatment time and solution temperature. A proper combination of these variables will permit many KMnO -H PO4 concentration combinations to be operative. Solutions containing from about 50% to by volume (54%7S% by wt. H PO of U.S.P. grade orthophosphoric acid, 5 g./l. to saturation of potassium permanganate, and the balance water, are effective for treatment times of less than about 30 minutes and operating temperatures of about 25 C. U.S.P. grade orthophosphoric acid contains -88% by weight H PO (usually about 85.3%) and the remainder H O. For purposes of this application, the expression U.S.P. orthophosphoric acid refers to the 85.3% by weight solution. Higher temperatures may, of course, be used effectively, though it is not normally necessary to do so. Higher temperatures tend to shorten the conditioning life of H PO --KMnO solutions. Longer treatment times may in some cases be effective. On the other hand, the possibility of a deleterious acid attack of the plastic increases with exposure time. The aforesaid range of solution concentrations permit room temperature operations at treatment times which vary from about 2-30 minutes. Particularly effective solutions comprise about 5-20 g./l. KMnO and 60-75% by volume (61%72% by wt. H PO of U.S.P. H PO and can be used for treatment times of about 16 minutes or less. A preferred solution comprises about 60% by volume (61% by wt. H PO U.S.P. phosphoric acid and 10 g./l. of potassium permanganate.

Cycolac test panels (3" x 3 /2) sold by Marbon Chemical Division of Borg Warner Corporation were used in a series of tests involving this invention. Cycolac is the name applied by Marbon Chemical Division to its plating grade ABS plastic and is further identified as 'Marbons ABS number BF-3510. The panels were first wiped clean with alcohol. These panels were conditioned in various KMNO H PO solutions at room temperature for periods which varied from 216 minutes. After conditioning, the panels were sensitized for 2 minutes, at room temperature, in a solution containing 50 grams of SnCl cc. of HCl, and 860 cc. of H 0. Subsequently, the panels were activated for 2 mintues, at room temperature, in a solution containing 264 milligrams of PdCl 5 cc. of HCl and cc. of H 0. An electroless copper coating was deposited on the panels for 5 minutes at room temperature. A proprietary bath known as Cuposit, which is sold by Shipley Co., Inc, of Newton, Mass., was used for this electroless copper coating. After the electroless copper coating, a copper strike was applied from a standard acid copper bath containing 225 g./l. of CuSO and 28.8 cc. per liter of H 80 An additional copper coating from another acid copper bath was deposited for 40 minutes in a half-box to a 2 mils thickness on the flat side of the test panel. The panels were then aged for two days and then adhesion tested by peeling one inch (1") wide test strips from the panels using the adhesion test procedure outlined in Marbon Chemical Divisions Technical Report No. P-103 regarding Quality Assurance ProgramPlastics, Cycolac BF-3510. The procedure calls for cutting a 1 wide strip on the plated test panel, forming a tab thereon and peeling the strip from the plastic using a tensometer While continuously recording the amount of pull required to peel the plate.

The adhesion of the plate to the ABS was measured in pounds per inch. For purposes of this application, adhesion data is reported as the mean between the readings on the test machine. The following are intended to serve as specific examples of our invention.

EXAMPLE 1 I A fresh solution containing g./l. KMnO 70% by volume (68% by wt. H PO of U.S.P. grade orthophosphoric acid and the balance Water was used to condition several panels for 16 minutes at 25 C. After plating and testing as outlined above, the panels were completely covered and displayed mean adhesion values of about 7.0 lbs/in.

EXAMPLE 2 A fresh solution containing 5 g./l. KMnO 80% by volume (75% by wt. H PO of U.S.P. orthophosphoric acid and the balance water was used to condition several panels for 8 minutes at 25 C. After plating and testing as outlined above, the panels were completely covered and displayed mean adhesion values of about 5.0 lbs/in.

EXAMPLE 3 A fresh solution saturated (21.4 g./l.) with KMnO 70% by volume (68% by wt. H PO of U.S.P. orthophosphoric acid and the balance water was used to condition several panels for 16 mintues at 25 C. After plating and testing as outlined above, the panels were completely covered and displayed mean adhesion values of about 7.8 lbs/in.

EXAMPLE 4 Several fresh solutions containing g./l. KMnO 60% by volume (61% by wt. H P0 of U.S.P. orthophosphoric acid and the balance water were used to condition several panels for from 4-16 minutes at 25 C. After plating and testing as outlined above, the panels were completely covered and displayed mean adhesion values of about 9.5 lbs/in.

EXAMPLE 5 Several fresh solutions containing g./l. KMnO 60% by volume (61% by wt. H PO of U.S.P. orthophosphoric acid and the balance water was used to condition several panels for from 2-16 minutes at C. After plating and testing as outlined above, the panels were completely covered and displayed mean adhesion values of about 7.2 lbs/in.

EXAMPLE 6 A fresh solution saturated (15.2 g./l.) with KMnO 80% by volume (75% by wt. H PO of U.S.P. orthophosphoric acid and the balance water was used to condition several panels for 8 minutes at 25 C. After plating and testing as outlined above, the panels were completely covered and displayed mean adhesion values of about 6.2 lbs/in.

During these tests, it was noted that panels treated with solutions having potassium permanganate concentrations of less than about 5 g./l. in 80% by volume (75% by wt. H PO U.S.P. orthophosphoric acid were adequately covered with deposited metal. However, solutions in this range had either a useful life of less than about two days, or the plate had a low adhesion value. In this particular series of tests, orthophosphoric acid concentrations of at least about 60% by volume (61% by wt. H PO consistently produced good results. During this series of tests, it was obseived generally that if the permanganate concentration was held constant, the mean adhesion values lessened as the H PO concentration of the solution was increased above about 60% by volume. It was likewise observed that below H PO, concentrations of about 70% by volume, the mean adhesion values lessened as the KMnO was increased above about 10 g./l. but that at concentrations above about 70% by volume (68% by wt. H PO this trend reversed itself.

Another series of tests were conducted wherein Cycolac EP-3510 test panels were treated for 30 minutes in a variety of KMNO -H PO solutions. These panels were sensitized for 1 minute at room temperature in a solution containing g./l. of SnCl -2H O, and 140 milliliters per liter of HCl. After sensitizing the panels were activated for 2 minutes in a room temperature solution containing 158 mg./liter of palladium chloride and 5 rnl./liter of HCl and then copper plated at room temperature for 40 minutes at 4 amps in a half-box to a deposit thickness of 2 mils. The following are some specific examples of the results of this procedure.

EXAMPLE 7 A fresh solution containing 10 g./l. KMnO 50% by volume (54% by wt. H PO of U.S.P. orthophosphoric acid and the balance water was used to condition several panels for 30 minutes at 25 C. After plating and testing as outlined above, the panels were completely covered and displayed mean adhesion values of about 9.9 lbs/in.

EXAMPLE 8 A fresh solution containing 2 g./l. KMnO 80% by volume (75 by wt. H PO of U.S.P. orthophosphoric acid and the balance water was used to condition several panels for 30 minutes at 27 C. After plating and testing as outlined above, the panels were completely covered and displayed mean adhesion values of about 6.4 lbs/in.

EXAMPLE 9 A fresh solution containing 5 g./l. KMnO 75% by volume (72% by wt. H PO of U.S.P. orthophosphoric acid and the balance water was used to condition several panels for 30 minutes at 25 C. After plating and testing as outlined above, the panels were completely covered and displayed mean adhesion values of about 9.8 lbs/in.

This latter series of tests also indicated that treatment with many combinations of KMnO and H PO will produce plateable panels. However, to accomplish this, more practical treatment times, solution temperatures, solution stability and adhesion values must frequently be sacrificed.

The amount of time required to properly treat the ABS is a function of several variables including the freshness of the solution, the respective concentration of the solutions ingredients, and the temperature of the solution. Generally when using solutions in the preferred range of about -75% (61%-72% by Wt. H PO U.S.P. H PO and 5-20 g./l. KMnO we prefer to treat the surface for about 8-16 minutes at room temperature.

In order to get the maximum useful life out of any particular bath, it is necessary to periodically add potassium permanganate thereto. Generally, the lower H PO concentration baths outlive the higher concentration baths. The KMnO H PO system is adaptable to the periodic addition of further KMnO without deleteriously affecting the solutions ability to function satisfactorily with respect to coverage, adhesion and life. In one particular case, a bath containing 10 g./l. of potassium permanganate and 60% by volume U.S.P. phosphoric acid had a sustained useful life of 5 days with a periodic potassium permanganate addition of only 42% of the original amount. This bath consistently produced plated samples having adhesions of from 6 to 8 lbs/inch. In another test the periodic addition of KMnO to a KMnO H PO solution sustained the operating life thereof for over 60 days. A comparison between the relative stability of the KMnO --H PO and KMnO H SO systems when no additions are made is shown in the following table. The table reflects the amount of KMnO remaining in solution after each day. The first day is considered to be the day the solutions were made up.

Grams/l. KMnO, remaining in acid 50% 80% 50% 60% 70% H3PO4 H3PO4 H 804 H 80 H 304 It should be noted that when adding KMnO to the solution, it should first be dissolved in H O to insure the maximum solubility of the KMnO Likewise normal care should be taken with respect to admixing of aqueous and acid solutions.

We have disclosed our invention primarily in terms of volumetric concentrations of U.S.P. orthophosphoric acid and weight concentrations of KMnO The handiest form of orthophosphoric acid is the U.S.P. grade 85.3% solution thereof. Likewise, the most readily available and handiest form of the permanganate is in the crystalline form. We appreciate that the solutions can actually be formulated using other materials which effect the same result. Thus since the permanganate ion is the principal oxidant it may be derived from other alkali metal salts thereof such as sodium permanganate. Likewise phosphoric anhydride, metaphosphoric acid and pyrophosphoric acid may be used to actually prepare the solution. These latter acids all form orthophosphoric acid in aqueous solutions. Therefore it is clear that it is the absolute concentration of the orthophosphoric acid and the permanganate ion that is significant rather than the particular means by which they are introduced into the solution. In this regard, the following information is presented. Solutions containing 50%, 60%, 70%, 75% and 80% by volume U.S.P. phosphoric acid (85.3% H PO contain approximately 54%, 61%, 68%, 72%, and 75% H PO by weight respectively. Likewise, the absolute permanganate ion concentration will vary depending on the phosphoric acid concentration. In this regard then a 5 g./l. concentration of potassium permanganate in an 80% by volume U.S.P. phosphoric acid solution yields about 0.23% by weight permanganate ion. A 5 g./l. concentration of KMnO in a 75% by volume solution of U.S.P. H3PO4 yields about 0.24% by weight permanganate ion, and 20 g./l. of KMnO in a 60% by volume U.S.P.

H PO solution yields about 1.04% by weight permanganate ion concentration.

Though we have disclosed our invention in terms of specific examples and preferred embodiments thereof, we do not intend to limit ourselves thereto except as defined by the claims which follow.

We claim:

1. A method of coating acrylonitrile-butadiene-styrene surfaces comprising the steps of conditioning said surfaces with a solution consisting essentially of about 54% to about 75% by weight orthophosphoric acid, an alkali metal permanganate sufiicient to produce a permanganate ion concentration of at least about 0.23% by weight and the balance water to render said surfaces more receptive to coatings deposited thereon and electrolessly depositing a metal coating onto said conditioned surfaces.

2. The method of claim 1 wherein said article is immersed in said solution and said alkali metal permanganate is potassium permanganate and wherein the concentration of said orthophosphoric acid is about 61% to about 72% by weight, the concentration of said potassium permanganate is sufiicient to produce a permanganate ion concentration of about 0.24% to about 1.04% by weight and the balance is water.

3. The method of claim 2 wherein the concentration of said orthophosphoric acid is approximately by weight, the potassium permanganate is sufiicient to produce a permanganate ion concentration of approximately 0.5% by weight and the balance is water.

4. The method of claim 2 wherein said article is immersed in said solution for about 4 to 16 minutes at about ambient temperature.

References Cited UNITED STATES PATENTS 3/1969 Kirkpatrick 156-2 5/1969 Klinger et al. 1l747 ALFRED L. LEAVITT, Primary Examiner J. A. BELL, Assistant Examiner 

